The direct use of sunlight to supply the basic human needs for energy is of primary importance for human survival on earth, since the stored fuels, fossilized or organic, are being consumed at an incredible rate.
When an object is exposed to solar radiation, its temperature rises until its heat losses become equal to its heat gains. The losses depend on the emission of radiation by the heated material, the movement of the surrounding colder air and the thermal conductivity of the materials in contact with it. The gains depend on the intensity of solar radiation and the absorptivity of solar radiation by its surface.
Solar radiation can be collected in two general ways to produce high temperatures, namely by covering a receiving surface with a sunlight transparent sheet of glass or plastic and by focusing the solar radiation from a large surface onto a receiver of small area.
With focusing collectors, it is easier to obtain much higher temperatures than with flat plate collectors, but they usually cost more to build and operate.